Biogeochemical processes of nitrate in Pétrola Basin: a saline lake-aquifer system.

Intense agricultural activities are among the main responsible for degradation of saline wetlands in semi-arid and arid climates due to elevated nutrient inputs. In 1998, the endorheic Pétrola Basin (High Segura River Basin, Central Spain) was declared vulnerable to nitrate (NO3-) pollution by the Regional Government of Castilla-La Mancha. This hypersaline lake was classified as a heavily modified waterbody due to the contribution of important inputs of pollutants from agricultural sources and urban waste waters, the latest are discharged directly into the lake without appropriate treatment, to the deterioration of the surface water body. An estimation of the NO3- mass reaching the lake from the surrounding streams and wastewater spill gives a figure of about 1.85 tn/year.
Earlier studies showed the two main flow components in the aquifer system: a regional groundwater flow from recharge areas into the lake and a density-driven flow from the lake to the underlying aquifer. The NO3- inputs are derived from nitrification of synthetic ammonium fertilizers, and afterwards, NO3- is expected to be attenuated around the lake by denitrification (up to 60%) in the saltwater-freshwater interface. Nevertheless, the spatial and temporal pattern of NO3- reduction in lake sediments is not known.
In this work, the main pathways for the NO3- attenuation linked to the sediment-water interface were clarified using isotope pairing techniques. For this purpose, mesocosms experiments were carried out: organic-rich lake sediment (up to 23% organic carbon content) was incubated for 96 hours with the addition of 15N nitrate tracer as KNO3. Two factors were modified during the experiments: light and oxic conditions. Analyzing inorganic N-species (n=20) over time (72 hours) showed that NO3 - attenuation was coupled with an increment in the ammonium (NH4+) concentration, from 0.8 mg/L up to 5.3 mg/L, and a decrease in redox values, from 135.1 mV up to -422 mV, in the water column.
In order to know the main microbial pathways in NO3- reduction, three different pathways were evaluated: denitrification, dissimilatory nitrate reduction to ammonium (DNRA) and anaerobic ammonium oxidation (Anammox). The complete reduction of 250 μmol of NO3- occurred during the first 30 hours of incubation under all the treatments, coupled to a temporal increase in the nitrite (NO2-) concentration. The outcomes will support the understanding how hypersaline lakes respond to elevated nutrient loads.